This invention relates to implantable medical devices (IMDs) and their various components, including flat electrolytic capacitors for same, and methods of making same, particularly such capacitors fabricated of a plurality of stacked capacitor layers each having anode layers fabricated of a plurality of anodized valve metal anode sheets.
A wide variety of IMDs are known in the art. Of particular interest are implantable cardioverter-defibrillators (ICDs) that deliver relatively high-energy cardioversion and/or defibrillation shocks to a patient""s heart when a malignant tachyarrhythmia, e.g., atrial or ventricular fibrillation, is detected. The shocks are developed by discharge of one or more high voltage electrolytic capacitor that is charged up from an ICD battery. Current ICDs typically possess single or dual chamber pacing capabilities for treating specified chronic or episodic atrial and/or ventricular bradycardia and tachycardia and were referred to previously as pacemaker/cardioverter/defibrillators (PCDs). Earlier developed automatic implantable defibrillators (AIDs) did not have cardioversion or pacing capabilities. For purposes of the present invention ICDs are understood to encompass all such IMDs having at least high voltage cardioversion and/or defibrillation capabilities.
Energy, volume, thickness and mass are critical features in the design of ICD implantable pulse generators (IPGs) that are coupled to the ICD leads. The battery(s) and high voltage capacitor(s) used to provide and accumulate the energy required for the cardioversion/defibrillation shocks have historically been relatively bulky and expensive. Presently, ICD IPGs typically have a volume of about 40 to about 60 cc, a thickness of about 13 mm to about 16 mm and a mass of approximately 100 grams.
It is desirable to reduce the volume, thickness and mass of such capacitors and ICD IPGs without reducing deliverable energy. Doing so is beneficial to patient comfort and minimizes complications due to erosion of tissue around the ICD IPG. The high voltage capacitor(s) are among the largest components that must be enclosed within the ICD IPG housing. Reductions in size of the capacitors may also allow for the balanced addition of volume to the battery, thereby increasing longevity of the ICD IPG, or balanced addition of new components, thereby adding functionality to the ICD IPG. It is also desirable to provide such ICD IPGs at low cost while retaining the highest level of performance. At the same time, reliability of the capacitors cannot be compromised.
Various types of flat and spiral-wound capacitors are known in the art, some examples of which are described as follows and/or may be found in the patents listed in Table 1 of commonly assigned U.S. Pat. No. 6,006,133. Typically, an electrolytic capacitor is fabricated with a capacitor case enclosing a xe2x80x9cvalve metalxe2x80x9d (e.g., aluminum) anode layer (or xe2x80x9celectrodexe2x80x9d), a valve metal (e.g. aluminum) cathode layer (or xe2x80x9celectrodexe2x80x9d), and a Kraft paper or fabric gauze spacer or separator impregnated with a solvent based liquid electrolyte interposed therebetween. The aluminum anode layer is typically fabricated from aluminium foil that is first etched and then xe2x80x9cformedxe2x80x9d by passage of electrical current through the anode layer to oxidize the etched surfaces so that the aluminium oxide functions as a dielectric layer. The electrolyte comprises an ion producing salt that is dissolved in a solvent and provides ionic electrical conductivity between the cathode layer and the aluminum oxide dielectric layer. The energy of the capacitor is stored In the electromagnetic field generated by opposing electrical charges separated by the aluminum oxide layer disposed on the surface of the anode layer and is proportional to the surface area of the etched aluminum anode layer. Thus, to minimize the overall volume of the capacitor one must maximize anode surface area per unit volume without increasing the capacitor""s overall (i.e., external) dimensions. The separator material, anode and cathode layer terminals, internal packaging, electrical interconnections, and alignment features and cathode material further increase the thickness and volume of a capacitor. Consequently, these and other components in a capacitor and the desired capacitance limit the extent to which its physical dimensions may be reduced.
Some ICD IPGs employ commercial photoflash capacitors similar to those described by Troup in xe2x80x9cImplantable Cardioverters and Defibrillators,xe2x80x9d Current Problems in Cardiology, Volume XIV, Number 12, Dec. 1989, Year Book Medical Publishers, Chicago, and as described in U.S. Pat. No. 4,254,775. The electrodes or anode and cathodes are wound into anode and cathode layers separated by separator layers of the spiral. Most commercial photoflash capacitors contain a core of separator paper intended to prevent brittle, highly etched aluminum anode foils from fracturing during winding of the anode, cathode, and separator layers into a coiled configuration. The cylindrical shape and paper core of commercial photoflash capacitors limits the volumetric packaging efficiency and thickness of an ICD IPG housing made using same.
More recently developed ICD IPGs employ one or more flat or xe2x80x9cprismaticxe2x80x9d, high voltage, electrolytic capacitor to overcome some of the packaging and volume disadvantages associated with cylindrical photoflash capacitors. Flat aluminum electrolytic capacitors for use in ICD IPGs have been disclosed, e.g., those Improvements described in xe2x80x9cHigh Energy Density Capacitors for Implantable Defibrillatorsxe2x80x9d presented by P. Lunsmann and D. MacFarlane at CARTS 96: 16th Capacitor and Resistor Technology Symposium, 11-15 Mar. 1996, and at CARTS-EUROPE 96: 10th European Passive Components Symposium., 7-11 Oct. 1996, pp. 35-39. Further features of flat electrolytic capacitors for use in ICD IPGs are disclosed in U.S. Pat. Nos. 4,942,501; 5,086,374; 5,131,388; 5,146,391; 5,153,820; 5,522,851, 5,562,801; 5,628,801; and 5,748,439, all issued to MacFarlane et al.
For example, U.S. Pat. Nos. 5,131,388 and 5,522,851 disclose a flat capacitor having a plurality of stacked capacitor layers each comprising an xe2x80x9celectrode stack subassemblyxe2x80x9d. Each capacitor layer contains one or more anode sheet forming an anode layer having an anode tab, a cathode sheet or layer having a cathode tab and a separator for separating the anode layer from the cathode layer.
Electrical performance of such electrolytic capacitors is affected by the surface area of the anode and cathode layers and also by the resistance associated with the electrolytic capacitor itself, called the equivalent series resistance (ESR). The ESR is a xe2x80x9chypotheticalxe2x80x9d series resistance that represents all energy losses of an electrolytic capacitor regardless of source. The ESR results in a longer charge time (or larger build factor) and lower discharge efficiency. Therefore, it is desirable to reduce the ESR to a minimum.
Typically, ESR is minimized by fabricating the anode layer of each capacitor layer from highly etched valve metal foil, e.g., aluminum foil, that has a microscopically contoured, etched surface with a high concentration of pores extending part way through the anode foil along with tunnels extending all the way through the anode foil (through-etched or tunnel-etched) or only with a high concentration of pores extending part way through the anode foil (nonthrough-etched). In either case, such a through-etched or nonthrough-etched anode sheet cut from such highly etched foil exhibit a total surface area much greater than its nominal (length times width) surface area. A surface area coefficient, the ratio of the microscopic true surface area to the macroscopic nominal area, may be as high as 100:1, which advantageously increases capacitance. Through-etched or tunnel-etched anode sheets exhibit a somewhat lower ratio due to the absence of a web or barrier surface closing the tunnel as in nonthrough-etched anode sheets.
After the aluminum foil is etched, the aluminum oxide layer on the etched surface is xe2x80x9cformedxe2x80x9d by applying voltage to the foil through an electrolyte such as boric acid or citric acid and water or other solutions familiar to those skilled in the state of the art. Typically, individual anode sheets are punched, stamped or otherwise cut out of the foil in a shape to conform to the capacitor package following formation of the aluminum oxide on the foil. The cut edges around the periphery of the anode sheets are carefully cleaned to remove particulates of anode material that can get caught between the capacitor layers in the electrode stack assembly resulting in a high leakage current or capacitor failure. Anode layers either comprise a single anode sheet or multiple anode sheets. Stacking the anode layer, s separator layers, and cathode layer together assembles capacitor layers, and electrode stack assemblies are assembled by stacking a plurality of capacitor layers together, separated by separator layers. The cut edges of the anode and cathode layers and any other exposed aluminum are then reformed in the capacitor during the aging process to reduce leakage current.
In order to increase capacitance (and energy density), multiple anode sheets are stacked together to form the multiple sheet anode layer as described above. Through-etched or tunnel-etched anode sheets need to be used In such multiple sheet anode layers to ensure that electrolyte is distributed over all of the aluminum oxide layers of the sandwiched inner anode sheets and to provide a path for ionic communication. But, then the gain in surface area is not as high as that which can be achieved with a like number of stacked non-through-etched anode sheets that have a remaining solid section in their center.
For example, the ""890 patent discloses the use of an anode layer fabricated from a highly etched center sheet with a solid core and two tunnel-etched anode sheets sandwiching the center sheet. This arrangement is intended to allow the electrolyte, and thus the conducting ions, to reach all surface areas of the three-sheet anode layer while preventing the ions from passing all the way through the anode layer. More than three tunnel etched anode sheets can be used in the anode layer depending on the desired electrical performance.
The aluminum oxide layers electrically Isolate the aluminum sheets of the aluminum layer from each other, and an electrical connection must be made between the underlying aluminum valve metal of each anode sheet of the anode layer. In one approach, each anode sheet of each anode layer is fabricated with an outwardly projecting anode tab. The tabs of the anode layers and the cathode layers of all of the capacitor layers of the stack are electrically connected in parallel to form a single capacitor or grouped to form a plurality of capacitors. The attached aluminum anode sheet tabs are electrically connected to a feedthrough pin of an anode feedthrough extending through the case or compartment wall. In the above-referenced ""851 patent, each of the anode sheet tabs are welded together and then welded to a post of a feedthrough pin. The single sheet cathode layers are also fabricated with cathode tabs that are also gathered together and electrically connected to a feedthrough pin of a cathode feedthrough extending through the case or compartment wall or connected to the electrically-conductive capacitor case wall.
Capacitor volume can be reduced slightly by interposing and welding a shared anode tab in between two adjacent anode sheets in the anode stack, as described, for example, in the above-referenced ""388 patent. No particular method of welding is disclosed, and the interposed stack of anode tabs would thicken and distort the anode sheet stack making it difficult to fit into a flatsided capacitor housing.
In another approach described in U.S. Pat. No. 5,584,890, the center anode sheet of a three-sheet anode layer is fabricated with an inward recess into which an anode tab is inserted. The three anode sheets are joined together at a distance from the anode tab by using cold welding, although laser welding and arc welding are mentioned as alternatives without detail.
In the above-referenced ""133 patent, a single anode tab is fitted into a slot of one of the stacked anode sheets and attached to one or more of the adjoining anode sheets by cold welding. The anode sheets are cold welded together at more than one location by use of a press and press fixture having spring-loaded or pneumatically driven cold weld pins that extend through pin bores of a top plate and a base plate bearing against the uppermost and lowermost exposed surfaces of the stack of anode sheets to be cold welded together.
By necessity, the joinder of anode sheets together to form multi-sheet anode layers and to separate anode tabs by such techniques must break through the oxide layer over the exposed etched surfaces of the anode sheets and fill or compress the underlying etched surface until the valve metals of the sheet cores are in intimate contact such that a low resistance electrical connection is achieved. Typically, it is necessary to provide multiple attachment sites to provide redundancy, which increases reliability. But breaking through the etched oxide layers of the multiple sheets in multiple places reduces the overall capacitance. Moreover, the attachment techniques can damage the etched oxide layers adjacent to the points of attachment or across the exposed outermost anodized surfaces of the outermost sheets of the anode layer.
Thus, there is a need for further reducing capacitor volume, increasing capacitor reliability, and reducing cost and complexity of the capacitor manufacturing process for high voltage electrolytic capacitors used in ICDs and other IMDs and other electric circuit applications.
The present invention provides for methods and apparatus for securely mechanically and electrically attaching anode sheets of multi-sheet anode layers of electrolytic capacitors together in a simple manner that minimizes the connection area and does not unduly damage formed surfaces of the anode sheets.
In accordance with the present invention, the side-by-side stacked multiple anode sheets of a multi-sheet anode layer are joined together by at least one malleable member that is fitted through a bore extending through each sheet and expanded in the aligned anode sheet bores to bear against the valve metal core layers of the anode sheets.
In a process in accordance with the invention, anode sheet bores are made through the anode sheets in the process of the sheet fabrication at locations that enable the axial alignment of the anode sheet bores into an anode layer bore when the anode sheets are stacked together to form an anode layer. The anode layer bore has an anode layer bore length corresponding to the sheet stack height of the side-by-side stacked anode sheets. A malleable member is inserted into each anode layer bore comprising the aligned anode sheet bores. Force is applied substantially only to the malleable member to compress it longitudinally and expand it laterally against the exposed valve metal of each anode sheet to effectively cold-weld the malleable member with the valve metal. In one sense, the expansion of the malleable conductive member functions as an expansion rivet, and these terms shall be used Interchangeably herein.
Preferably, the malleable conductive member comprises a pin fitted through the aligned anode sheet bores, and the axially applied forces longitudinally compress and laterally expand the pin to interference fit against the valve metal of the anode sheets. The pin and the anode sheet bores forming the anode layer bore have a compatible cross-section shape so that the pin can be inserted into the anode layer bore. The pin length preferably exceeds the anode layer bore length by a predetermined amount. The pin length and the bore size and the cross-section size of the pin are selected so that the pin expansion normal to the applied force is sufficient to apply welding force against the anode sheets to weld them to the malleable member thereby making a unitary anode layer without fracturing or distorting the anode sheets. In the process, the pin is shortened until the pin length equals the anode layer bore length and is flush with the outermost surfaces of the anode layer, that is, the outer exposed surfaces of the outer anode sheet of the anode.
The bore and pin are preferably circular. If the bore is non-cylindrical, the pin is preferably keyed in cross-section shape to fit the cross-section shape of the aligned anode sheet bores or otherwise dimensioned to fit the cross-section shape of the aligned anode sheet bores.
Or the malleable conductive member can comprise a plurality of malleable pellets fitted through the aligned first and second bores such that the stacked pellet height in the aligned anode sheet bores exceeds the stack height and anode layer bore length of the side-by-side stacked anode sheets by a predetermined amount. Then, the malleable pellets are compressed by force applied axially thereto so that the pellets are welded together and are substantially flush with the outermost surfaces of the anode layer. The stacked pellet height, the bore and pellet sizes, and the cross-section shapes are selected so that the pellet expansion normal to the applied force is sufficient to apply sufficient force against the anode sheets to weld the valve metal core layers of the anode sheets to the welded together stack of pellets, thereby making a unitary anode layer without fracturing or distorting the anode sheets.
The bore is preferably circular and the malleable pellets are preferably cylindrical or spherical. If the bore Is non-cylindrical, the malleable pellets are keyed to or otherwise fit the cross-section shape of the aligned anode sheet bores.
The malleable conductive member and the anode sheets are preferably fabricated from the same valve metal, e.g., aluminum. Preferably, the anode sheets of the anode layer are attached using a plurality of such bores and malleable conductive members.
The method of the invention is preferably also employed to fix an anode tab to the anode sheets of the anode layer. At least one anode sheet is fabricated having a slot or notch into which the anode tab is fitted. The remaining anode sheets are fabricated with sheet bores that are axially aligned to extend across or traverse the notch when the anode sheets are stacked together. The anode tab is fabricated with a tab bore that is aligned with the aligned anode sheet bores that traverse the notch when the anode tab is inserted into the notch. The malleable member is then inserted into the aligned sheet and tab bores and compressed to affix the anode sheets and tab together.
The resulting anode layer of the present invention then comprises a first anode sheet formed of a valve metal having first and second sheet sides that are bounded by a sheet edge, the first anode sheet having a first bore extending from the first sheet side to the second sheet side and at least one second anode sheet formed of a valve metal having first and second sheet sides that are bounded by a sheet edge, the first anode sheet having a second bore extending from the first sheet side to the second sheet side. A malleable conductive member or expansion rivet is interference fitted through the first and second bores and into physical and electrical contact with the valve metal of the first and second sheets, whereby the valve metal of the first and second sheets are electrically and mechanically connected together.
Preferably, the anode layer further comprises an anode tab fitted In a notch of the first anode sheet and attached to the second anode sheet by a malleable member traversing a third bore through the second anode sheet and a tab bore traversing the anode tab and axially aligned with the third bore.
Advantageously, any number of two or more anode sheets can be coupled together following the teachings of the present invention. A robust electrical and mechanical connection of the anode sheets of the anode layer is achieved through the present invention. In addition, compressive forces applied to the malleable conductive members only compress and expand the malleable conductive members in the aligned anode sheet bores. Compression and damage of the etched and anodized layers of the anode sheets is minimized as applied force is confined substantially to compression of the malleable conductive members, and a high capacitance per unit area is achieved.
A capacitor is assembled from the anode layer, a cathode layer, and a separator between the anode layer and the cathode layer and fitted into a capacitor case with appropriate electrical connections made from the anode and cathode tabs to respective anode and cathode terminals of the capacitor. Or, a capacitor layer is assembled from the anode layer, a cathode layer, and a separator between the anode layer and the cathode layer, a plurality of the capacitor layers are stacked into a capacitor sub-assembly and fitted into a capacitor case, and the anode and cathode tabs of the capacitor layers are electrically interconnected to anode and cathode terminals of the capacitor.
In one embodiment, an exemplary electrolytic capacitor formed in accordance with the present invention comprises an electrode stack assembly and electrolyte located within the interior case chamber of a hermetically sealed capacitor case. The electrode stack assembly comprises a plurality of capacitor layers stacked in registration upon one another, each capacitor layer comprising a cathode layer having a cathode tab, an anode layer comprising at least one anode sheet having an anode tab, and a separator layer located between adjacent anode and cathode layers, whereby all adjacent cathode layers and anode layers of the stack are electrically insulated from one another by a separator layer. Anode terminal means extend through the capacitor case sidewall for electrically connecting a plurality of the anode tabs to one another and providing an anode connection terminal at the exterior of the case. Cathode terminal means extend through or to an encapsulation area of the capacitor case side wall for electrically connecting a plurality of the cathode tabs to one another and providing a cathode connection terminal at the exterior of the case. A connector assembly is electrically attached to the anode connection terminal for making electrical connection with the anode tabs and to the cathode connection terminal for making electrical connection with the cathode tabs.
This summary of the invention and the advantages and features thereof have been presented here simply to point out some of the ways that the invention overcomes difficulties presented in the prior art and to distinguish the invention from the prior art and is not intended to operate in any manner as a limitation on the interpretation of claims that are presented initially in the patent application and that are ultimately granted.